Project Summary Melanoma is the deadliest form of skin cancer. We aim to understand mechanisms underlying oncogenic signaling and drug resistance in melanoma in order to improve the current treatment options. The serine/threonine kinase, BRAF, is somatically mutated in ~50% of melanomas and drives hyper-activation of MEK-ERK1/2 signaling, aberrant growth and invasion. While BRAF and MEK inhibitors are now first-line therapy for advanced-stage mutant V600E BRAF melanomas, they only delay mortality. Thus, we must understand how to build on BRAF+MEK inhibitor treatments and determine how to combine them with immune checkpoint agents, which remove the blocks on T cell action. In the previous cycle of this grant, we showed that up-regulation of FOXD3- ErbB3 signaling is an adaptive response to BRAF inhibitors that promotes a state of drug tolerance in mutant BRAF melanoma. Furthermore, we showed that inhibition of ErbB3 enhanced the effects of BRAF inhibitors in mutant BRAF melanomas in vivo. In this current proposal, we aim to identify mechanisms regulating FOXD3 in mutant BRAF melanomas, test the possibility that epigenetic BET/BRD inhibitors will block adaptive responses to BRAF+MEK inhibitors and alter the tumor microenvironment, and determine how FOXD3 expression alters the infiltration/activation status of tumor-associated immune cell populations and the expression of putative immune checkpoint proteins. At the completion of our experiments, we expect to have identified tumor and stromal markers for adaptive responses to targeted therapy and informed future combinatorial targeted inhibitor, epigenetic inhibitor and immune-based therapeutic strategies in mutant BRAF melanoma.